1. Field of the Invention
This invention relates generally to annular blowout preventers and more particularly to a packing unit and insert for such packing unit. More particularly, the invention relates to an insert for an annular packing unit for imparting significantly longer cycle life to the packing unit when it is periodicly tested during drilling operations.
2. Description of the Prior Art
For many years the design of blowout preventer packing units has followed the principles described in U.S. Pat. No. 2,609,836 to Knox. Such packing units incorporate metal inserts equally spaced about the packer longitudinal axis and embedded in an elastomeric material. Upon inward constriction or closure of the unit about a well drill pipe, the elastomeric material is anchored by the insert webs as it produces vertical folds stretching radially inwardly to seal the pipe. When the packer unit closes on itself as in the case when no pipe is present in the longitudinal bore of the unit, elastomeric material of the folds advancing toward the axis must at certain times stretch or extend as much as 350 to 400%. Repeated closures produce excessive wear and fatigue of the elastomeric or rubber material, reducing the useful life of the packer due to such extreme stretching.
U.S. Pat. Nos. 3,958,808 and 3,917,293 disclose a packing unit and inserts for such a packing unit which significantly increase the packing unit cycle lifetime. The increase in cycle lifetime results from locating the insert webs in the elastomeric material with spacing from the central longitudinal axis and from each other in a differential manner in order to provide differential anchoring about the axis of circularly spaced portions of the elastomeric material subject to inward displacement. Rubber flow paths are opened between the inserts.
In one form of the invention disclosed in the above referenced patents, the anchoring of the rubber by certain webs is closer to the central axis than anchoring of the material by other webs. In other words, webs effectively closer to and further from the axis may alternate about the axis. In other forms of the invention, the webs are generally equally spaced from the axis but certain pairs of the webs have closer circular spacing about that axis than other pairs of webs. Significant increases in packer cycle life are achieved by the invention disclosed in the above referenced patents. A continuing need exists, however, for increases in packer cycle life over and above those achieved by the above referenced inventions.
Thus, it is an object of the invention to provide a packer and an insert designed for the packer which produces increased packer unit cycle life.
It is another object of the invention to provide a packer and an insert for use therein which produces a reversing of the pressure profile on the pipe moving the peak pressure to the lower end of the packing unit thereby producing less extrusion about the top plate of the packing unit, with resulting increase in cycle life of the packer unit.
It is a further object of the invention to provide an insert for an annular packing unit in which the elastomeric material in which the insert is embedded is placed in compression at the top portion of the web connecting the upper plate with the lower plate thereby tending to force the elastomeric material toward the web when the packer unit is closing on a pipe or other object in the packer unit bore, thereby reducing tearing and extrusion above the top plate of the packing unit.